Hot melt adhesives typically exist as solid masses at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing a variety of disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, feminine sanitary napkins, pantyshields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate. A second substrate is immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the absence of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating the costly process associated with solvent removal.
For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film by using piston or gear pump equipment. In this case, the substrate is brought into intimate contact with a hot die under pressure. The temperature of the die must be maintained well above the melting point of the adhesive, which is typically between 150 and 200° C. For some applications, particularly for manufacturing nonwoven articles, bonding of delicate and heat sensitive substrates, such as thin gauge polyethylene films, is often involved. Direct contact between the film and the die, in these cases, must be avoided to prevent the film from burning or distorting. Several application methods have been developed through which a hot melt adhesive can be spray coated with the aid of compressed air onto a substrate from a distance. These non-contact coating techniques include spiral spray and various forms of melt-blown methods. Direct contact between the coating head and the substrate is thus eliminated. All the coating techniques herein described above are well known to those skilled in the art and commercial equipment is readily available.
The spray coating techniques, however, pose stringent requirements on hot melt adhesives. The viscosity of the adhesives must be sufficiently low, usually in the range of 2,000 to 30,000 cP, preferably in the range of 2,000 to 15,000 cP, at the application temperature. Many other physical factors, especially the rheological properties of the adhesive, come into play in determining the sprayability of a hot melt. The majority of commercial hot melt products do not lend themselves to spray applications. There are no accepted theoretical models or guidelines to predict sprayability and it must be determined empirically with application equipment.
Syndiotactic polypropylene (SPP) polymers are known in this art. The SPP polymers are essentially high molecular weight stereospecific propylene homopolymers or copolymers of propylene with other α-olefin monomers such as ethylene, butene-1 or hexene-1. The syndiotactic polymers should not be confused with the conventional crystalline polypropylenes and essentially amorphous atactic poly-α-olefins (APAO). These polymers differ from each other in both structures and properties. It is well know to those skilled in the art that the conventional crystalline polypropylenes have an isotactic molecular chain configuration. The isotactic configuration can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane drawn through the main polymer chain. This type of stereochemistry structure can be illustrated graphically by using the Fisher projection formula as the follow:
Due to its high degree of chain regularity, the conventional isotactic polypropylenes (IPP) are highly crystalline with crystallinity typically greater than 50% and a heat of fusion greater than 70 J/g. They are usually stiff materials having high density and high melting point. Due to the lack of flexibility, an IPP polymer can only be used as a modifier in small amounts, typically around 2% to 5% by weight, in hot melt adhesive formulations. A typical conventional IPP usually has a melt flow rate, which is inversely related to the weight average molecular weight, in the range of 0.5 to 200 g/10 min as measured in accordance with ASTM D-1238 test method.
Another component known to be used in a hot melt adhesive composition blend comprises an APAO polymer. APAO polymers are a family of essentially amorphous low molecular weight homopolymers of propylene or copolymers of propylene with ethylene or butene or hexene. In contrast to the regular structures in IPP or SPP, APAOs have atactic molecular chains with the methyl groups on the successive monomeric units sterically randomly distributed on both sides of the hypothetical plane through the polymer chain. The stereo configuration of the atactic APAO molecular chain can be illustrated graphically by using the following Fisher projection formula:

The stereo chain structure of SPP is uniquely different from that of IPP and from that of APAO. In contrast to the isotactic chain configuration of IPP and the atactic chain configuration of APAO, the stereochemistry of SPP can be described as having the tertiary methyl groups of successive monomer units along the chain alternatively disposed on each side of the hypothetical plane. The stereo configuration of SPP can be depicted below:

The stereo configuration of polypropylene can also be characterized quantitatively through C-13 NMR. In NMR nomenclature, a “meso” dyad of successive methyl groups on the same side of the plane, as in the case of IPP, is represented by the letter m. A “racemic” dyad of successive methyl groups on the opposite sides of the plane, as in the case of SPP, is represented by the letter r. The percentage of m or r defines the degree of polymer tacticity with the sum of m and r equal to 100%. Thus, a perfect isotactic polypropylene will have 100% m dyad, whereas a perfect syndiotactic polypropylene will have 100% r dyad. This unique stereochemical structure of SPP results in an unusual and desirable combination of physical and mechanical properties such as low density, low melting point, flexibility and toughness. SPP polymers typically have an r value equal to or greater than 70% while the r values of conventional IPPs, in comparison, are generally in a few percent range.
In addition to the difference in stereochemistry, SPPs are also readily distinguishable from IPPs and APAOs by their unique physical properties. Typical SPPs will have a melting point between 130 to 160° C., whereas crystalline IPPs typically have a melting point about 176° C. APAOs, on the other hand, are usually predominately amorphous without a well-defined melting point although some grades of commercial products may exhibit very low degree of crystallinity. Another profound difference between SPP and IPP and APAO lies in their densities. The density of SPP is typically between 0.86 to 0.90 g/cm3, which is in between those of IPP and APAO. IPPs have the highest density ranging from 0.90 to 0.95 g/cm3 and APAOs, the lowest ranging from 0.85 to 0.87 g/cm3.
Due to their high melting point, high degree of crystallinity and the lack of desirable physical and mechanical attributes such as flexibility and toughness, the conventional IPPs have not been used alone as the polymer base for hot melt adhesive applications. A hot melt adhesive based on IPP would be too brittle to offer acceptable bond strength and yet would require high application temperature that goes well beyond the melting point of the polymer.
Hot melt adhesives containing APAOs, APAO/polyethylene (PE) blends, APAO/polybutene (PB) blends, or APAO/IPP blends are known in the art. These adhesives typically consist of an APAO, or an APAO blend herein mentioned above, and a hydrocarbon type of tackifier. It is well know that adhesives based on APAOs generally have poor cohesive strength, poor heat resistance, low elevated temperature bond strength and low shear values. APAOs have not found much use in disposable nonwovens applications where a combination of high bond strength at very low coating weight and easy processibility by spray techniques is required. The APAO based adhesives usually lack such capabilities. Although various attempts were made to address these problems by blending APAO with PE, PB and the conventional IPP, very often such modifications not only failed to rectify the problems, but also led to adverse side effects.
For example, Trotter et al, in U.S. Pat. No. 4,022,728, describes a hot melt pressure sensitive composition comprising a mixture of APAOs, a low molecular weight substantially amorphous elastomer, a liquid tackifier and a conventional crystalline polypropylene (IPP) in the amount of up to 2% by weight. It is claimed that the composition provides good adhesive properties at low temperatures.
Meyer et al, in U.S. Pat. No. 4,120,916, discloses hot melt adhesive compositions comprising a blend of low molecular weight polyethylene, low molecular weight conventional propylene containing polymer and APAO. These adhesive compositions are said to offer short open time and to be useful for bonding of paraffin modified corrugated board.
Lakshmanan et al, in U.S. Pat. No. 4,761,450, discloses a polymer blend useful as hot melt adhesive comprising a low density ethylene polymer, a copolymer of butene-1 with ethylene or propylene, a hydrocarbon tackifier and a low molecular weight polymer consisting of a low molecular weight liquid polybutene, an amorphous polypropylene and mixtures thereof.
Lakshmanan et al, in U.S. Pat. No. 5,478,891, also discloses blend compositions containing (a) a high molecular weight copolymer of ethylene with an α-olefin having at least 4 carbons and (b) an amorphous polypropylene or amorphous polyolefin. The components of the blends are described as having molecular weight range between 300 to 6000. The polymer blends are claimed to be useful for hot melt adhesives, coatings, sealants, asphalt modifiers and plastic additives.
Ryan discloses in U.S. Pat. No. 5,747,573 an APAO based hot melt adhesive composition useful for bonding plastics and metallized foil containers. The adhesive composition contains a blend of APAO, a solid benzoate plasticizer and a hydrocarbon tackifier.
Sustic, in U.S. Pat. No. 5,723,546, discloses a polymer blend consisting of a high molecular weight average, predominantly atactic flexible polyolefin polymer and a low molecular weight average APAO. The blend is said to be useful for hot melt adhesives.
Blending APAO with PE, PB or the conventional IPP leads to several drawbacks. The prior art adhesives containing APAO/PE or APAO/PB blends, such as, for example, those described herein above in U.S. Pat. Nos. 4,120,916, 4,761,450, and 5,478,891, tend to have poor compatibility. These adhesives can undergo phase separation during application process at which the hot melt adhesives have to be kept in the molten state at high temperature for a prolonged period of time, sometimes for hours or even days. Charring, skinning and gelling can develop rather quickly in the phase separated hot melt adhesives, thereby causing the application equipment to block or plug-up. The incompatibility of such polymer blends also imparts brittleness, optical haziness, poor or no open time, and low bond strength. Although APAO and the conventional IPP blend based hot melts do not have the compatibility problems, they still suffer from all the other drawbacks herein described above. Moreover, due to high crystallinity and high melting point of the conventional IPP polymers, hot melt adhesives based on APAO/IPP blends tend to be hard and brittle unless the IPP polymer amount is kept at a very low level, such as, for example, at about or below 2% by weight as disclosed in the prior art U.S. Pat. No. 4,022,728. As a result, these adhesives will have poor tensile strength, poor bond strength and poor impact resistance. Another detrimental effect of IPP is the increased coating temperature. The adhesive must be heated above the melting point of IPP (ranging from 180 to 200° C.) for it to reach a liquid state. The blend of high and low molecular weight atactic polyolefin approach described in U.S. Pat. No. 5,723,546, although offering some improvement on tensile properties of APAO, has not been able to provide sufficient tensile strength and high temperature properties to overcome the deficiencies of sole APAO based hot melts.
In a prior U.S. Pat. No. 5,317,070, Brant et al disclosed a hot melt adhesive based on tackified SPP having a polymer chain of at least 80% racemic dyads and having a melting point of about 100 to 180° C. The adhesive is claimed to have good open times between the application of the adhesive and the formation of the joint. This type of tackified SPP usually lacks flexibility and toughness, and therefore, will also have poor bond strength and poor impact resistance. Furthermore, SPP exhibits an inherent shrinkage problem when it transforms from liquid to solid crystalline state. The shrinkage often causes stress concentration at adhesive/substrate interfaces, and consequently, catastrophic bond failure.
It therefore would be advantageous to provide a hot melt adhesive that will overcome the shortcomings of the prior art adhesives herein mentioned above.
With the advancement in recent years of catalyst technology in the manufacturing of polyolefins, especially with the newly developed single-site catalyst systems such as metallocenes, entirely new types of flexible, low melting point and low crystallinity random propylene copolymers (RCP) have been developed. The art of single-site metallocene catalysts is a subject of numerous publications, such as, for example, U.S. Pat. Nos. 5,387,568, 5,393,851, 5,416,228, 5,476,914 to Ewen et al, and U.S. Pat. No. 5,789,502 to Shamshoun et al. Compared with conventional IPPs, the RCP copolymers usually have better flexibility, better impact resistance, lower density, much depressed melting point and lower crystallinity; these are the characteristics favoring hot melt adhesive applications. It is discovered in the present invention that RCP copolymers based on propylene and α-olefins can be advantageously used to overcome the drawbacks of IPP, SPP and APAO in hot melt adhesives to provide well balanced properties of cohesive strength and adhesive strength. Most importantly, the present invention provides a hot melt composition that can be easily processed with a variety of conventional hot melt coating equipment.